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Introduction

KAT5 was originally identified in 1996 as a cellular protein interacting with the HIV-1 Tat transactivator, leading to its designation as Tip60 (Tat-interactive protein, 60 kDa) [1]. The human KAT5 gene is located on chromosome 11q13 and encodes a 513-amino acid protein. Its primary histone substrates are histone H4 lysines 5, 8, 12, and 16 (H4K5/8/12/16ac) and histone H2A lysine 5 (H2AK5ac), marks that promote chromatin relaxation and transcriptional activation.

KAT5 is the catalytic subunit of the NuA4/TIP60 multiprotein complex, one of the most evolutionarily conserved chromatin-modifying assemblies in eukaryotes. The yeast NuA4 complex was first characterized biochemically in 1999 as an essential transcription adaptor and histone H4 acetyltransferase containing Esa1p and the ATM-related cofactor Tra1p [2]. The human NuA4/TIP60 complex comprises more than 16 subunits, including the chromatin remodeling ATPase EP400, the scaffolding protein TRRAP, and multiple subunits shared with the SWR1 complex. High-resolution cryo-EM structural analysis of the intact human complex has recently revealed the architectural basis for its dual acetyltransferase and chromatin remodeling activities [3].

Section 2 — Structure and Biochemistry

KAT5 contains a chromodomain at its N-terminus that recognizes trimethylated histone H3 lysine 9 (H3K9me3). The central MYST domain harbors the acetyltransferase active site, with an essential cysteine residue in the zinc finger contributing to catalysis. Structural analysis of the intact NuA4/TIP60 complex has revealed how the multiple subunits are organized into functional modules that coordinate histone acetylation with ATP-dependent nucleosome remodeling and histone variant H2A.Z deposition [3].

A critical regulatory mechanism involves acetylation of KAT5 itself at lysine 327 by CBP/CREBBP, which enhances KAT5 activity and is required for the full DNA damage response [4]. Conversely, deacetylation of KAT5 by SIRT1 attenuates its activity, providing a dynamic regulatory circuit coupling metabolic state to DNA repair capacity.

Section 3 — Role in DNA Damage Response

KAT5 plays an essential and early role in the cellular response to DNA double-strand breaks (DSBs). Within minutes of DSB induction, KAT5 is recruited to damage sites and acetylates histone H4K16, displacing the linker histone H1 and relaxing chromatin to facilitate access of repair factors [4]. Critically, KAT5 also directly acetylates and activates ATM kinase at lysine 3016, a modification required for full ATM activation and subsequent phosphorylation of H2AX, CHK2, and p53 [5]. KAT5 additionally acetylates p53 at lysine 120, enhancing p53-dependent transcription of pro-apoptotic targets [6].

Section 4 — Role in Transcription and Development

The NuA4/TIP60 complex is recruited to active gene promoters through interactions with transcription factors including MYC, E2F, and nuclear receptors [2]. At these sites, KAT5-mediated H4 acetylation and H2A.Z deposition maintain nucleosome-depleted regions and facilitate RNA polymerase II elongation. KAT5 is essential for early embryonic development: homozygous Kat5-null mouse embryos die at the blastocyst stage, reflecting a requirement for KAT5 in inner cell mass formation and pluripotency maintenance. In embryonic stem cells, KAT5 co-occupies enhancers with OCT4, SOX2, and NANOG, and its loss leads to rapid differentiation.

Section 5 — KAT5 in Cancer

KAT5 functions as a haploinsufficient tumor suppressor. Reduced KAT5 expression is documented in prostate cancer, breast cancer, colorectal cancer, and head and neck squamous cell carcinoma, where it correlates with advanced stage and poor prognosis [7]. The tumor suppressive function of KAT5 is attributed to its roles in activating ATM-p53 signaling, suppressing MYC target gene expression, and maintaining genome stability [8]. Small-molecule inhibitors targeting the KAT5 bromodomain or acetyltransferase active site, including TH1834 and NU9056, show activity in prostate cancer models.

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